1. Field of the Invention
The present invention relates to a permanent-magnet rotating machine, such as an electric motor, and in particular, to a permanent-magnet rotating machine designed to achieve a reduction in cogging torque.
2. Description of the Background Art
In an ordinary construction of a permanent-magnet rotating machine, a rotor is disposed inside a stator. The stator has a tubular iron core carrying a plurality of stator coils arranged on its curved inner surface to form multiple magnetic poles. The rotor has a rotor core and shaft placed inside the stator so that the rotor can rotate about a central axis of the stator. Permanent magnets are provided on the curved outer surface of the rotor core or embedded in it. The permanent magnets are arranged in such a way that their north (N) and south (S) poles alternate along the surface of the rotor core. The rotating machine causes electric currents to flow through the stator coils to produce a rotating magnetic field so that the rotor rotates about its shaft.
In the rotating machine thus constructed, there occur variations in revolving torque and speed. This phenomenon known as “cogging” causes not only vibrations and noise but also deterioration of controllability of the rotating machine.
Japanese Laid-open Utility Model Publication No. 1986-17876, for example, discloses an arrangement for reducing the cogging torque. According to the Publication, multiple rows of permanent magnets are arranged on a cylindrical surface of a rotor core along its axial direction in a manner that the permanent magnets are offset, or skewed, in the circumferential direction of the rotor core to produce a skewing effect. More specifically, the multiple permanent magnets are skewed in the circumferential direction from one row to next, according to their location along the axial direction of the rotor core, such that the permanent magnets are arranged on the surface of the rotor core at a skew angle (hereinafter referred to as the row-to-row skew angle) θm.
Conventionally, a theoretically determined angle (hereinafter referred to as the theoretical angle) is used as the row-to-row skew angle (physical angle) θm. The theoretical angle at which the cogging torque is expected to be minimized is calculated as 360/(the smallest number of which the number of stator poles and the number of rotor poles are factors)/(the number of permanent magnet rows along the axial direction) as discussed in Japanese Laid-open Patent Publication No. 2000-308286, for example.
As an example, if the number of stator poles of a rotating machine is 12, the number of rotor poles is 8, and the number of permanent magnet rows along the axial direction is 4, and it is intended to reduce the cogging torque individually by the upper two rows and lower two rows of permanent magnets, the row-to-row skew angle θm of the upper two rows, and of the lower two rows, is 7.5 degrees (=360/24/2, which is not 30 degrees in electrical angle θe).
Japanese Patent Publication No. 2672178 and Japanese Laid-open Patent Publication No. 1996-251847, for example, disclose another arrangement for reducing the cogging torque. Specifically, the number of permanent magnet rows is set to 2n, where n is an integer equal to 2 or larger, or permanent magnets are attached in nonuniform positions.
Even if the theoretically determined row-to-row skew angle θm is applied to an actual rotating machine, however, it is considered still insufficient for reducing the cogging torque. This is because the influence of magnetic saturation due to magnet flux leakage, which is caused by the aforementioned skewed magnet row arrangement, is not taken into consideration. While a leakage flux that causes the cogging torque could occur at joints between the permanent magnet rows and on the interior of the rotor core, for instance, a leakage flux occurring inside the stator core is a major cause of the cogging torque.
As stated above, the conventional skewed magnet row arrangement used in the rotating machine is associated with the problem that the cogging torque can not be reduced sufficiently since the theoretically determined skew angle is used the row-to-row skew angle.